A substantial amount of research and development is being done to reduce our dependency on petroleum-based energy and to move us toward more sustainable energy sources, such as wind energy, solar energy, and biomass. Of these three sustainable energy sources, biomass is receiving the most attention. One reason for this is that biomass is widely available in a variety of forms. Also, the economy of agricultural regions growing crops can greatly benefit from plants that convert biomass to transportation fuels. Thus, governmental bodies in agricultural regions are supportive of proposed biomass plants. In order to convert biomass to high value products, such as transportation fuels, it typically must first be converted to a synthesis gas (syngas) by gasification. Biomass gasification is a well-known process for producing syngas, which is a gas mixture containing varying amounts of carbon monoxide and hydrogen as the major components.
Syngas leaving the gasifier must be cooled and cleaned before it is suitable as feedstock for chemical synthesis. It contains various constituents that can foul downstream equipment, damage compressors and poison catalysts used in downstream processes. Solids entrained in the syngas are typically fines that are generated from the attrition of solids circulating in the gasifier, ash generated from the biomass, and soot. Furthermore, syngas derived from biomass generally contain relatively large amounts of CO2 compared to syngas derived from natural gas or coal. This is a consequence of the low heating value of biomass as well as the process conditions needed to produce a syngas having the desired H2 to CO ratio for downstream processing, such as for the production of higher value liquid products. The overall yield of desired liquid product from syngas can be increased by removing as much of the CO2 as possible. Large amounts of CO2 in the syngas make conventional acid gas removal technologies, such as, caustic scrubbing, economically infeasible. Other commercially available technologies based on physical absorption to remove CO2 from syngas streams are also not economically feasible. For example, the use of methanol and amine scrubbing (MEA, MDEA) are not particularly suitable for the removal of CO2 from syngas generated from biomass because they are energy intensive requiring large refrigeration loads and reboiler duties, respectively.
While commercial processes presently exist for removing CO2 from gaseous mixtures, they are typically not economically suitable for removing relatively high levels of CO2 from syngas streams. Therefore, there is a need in the art for a more cost effective technology capable of removing CO2 from syngas streams containing at least 5 vol. % CO2.